Single-Mode Optical Fiber Having Reduced Bending Losses

ABSTRACT

A single-mode optical fiber includes a central core, an intermediate cladding, a depressed trench, and an external optical cladding. The central core has a radius r 1  and a positive refractive index difference Δn 1  with the optical cladding. The intermediate cladding has a radius r 2  and a positive refractive index difference Δn 2  with the optical cladding, wherein Δn 2  is less than Δn 1 . The depressed trench has a radius r 3  and a negative index difference Δn 3  with the optical cladding. At a wavelength of 1310 nanometers, the optical fiber has a mode field diameter (MFD) between 8.6 microns and 9.5 microns and, at a wavelength of 1550 nanometers, the optical fiber has bending losses less than about 0.25×10 −3  dB/turn for a radius of curvature of 15 millimeters. At a wavelength of 1260 nanometers, attenuation of the LP11 mode to 19.3 dB is achieved over less than 90 meters of fiber.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application claims the benefit of pending French Application Ser.No. 08/02503 for “Fibre Optique Monomode” (filed May 6, 2008, at theFrench Patent Office), which is hereby incorporated by reference in itsentirety.

This application further claims the benefit of U.S. Patent ApplicationNo. 61/101,337 for a “Bend-Insensitive Optical Fiber” (filed Sep. 30,2008), U.S. Patent Application No. 61/112,006 for a “Bend-InsensitiveSingle-Mode Optical Fiber” (filed Nov. 6, 2008), and U.S. PatentApplication No. 61/112,374 for a “Bend-Insensitive Single-Mode OpticalFiber” (filed Nov. 7, 2008), each of which is incorporated by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates to the field of optical fibertransmissions, and more specifically, to an optical fiber having greatlyreduced bending losses.

BACKGROUND OF THE INVENTION

For optical fibers, the refractive index profile is generally set forthin terms of the difference in value between two points on the graph ofthe function associating the refractive index with the radius of thefiber. Conventionally, the distance r to the center of the fiber isshown along the x-axis of the profile. The difference between therefractive index at distance r and the refractive index of the externalfiber cladding is shown along the y-axis (FIG. 2, references 21-24). Theexternal cladding functions as an optical cladding and has asubstantially constant refractive index. This optical cladding isgenerally composed of pure silica but can also contain one or moredopants. The optical fiber refractive index profile is referred to as a“step” profile, a “trapezoidal” profile, or a “triangular” profile forgraphs having the respective shapes of a step, a trapezoid, or atriangle. These curves are generally representative of the theoreticalor reference index profile (i.e., set profile) of the fiber. Fibermanufacturing constraints may lead to a slightly different profile inthe actual fiber.

An optical fiber is conventionally composed of (i) an optical core,having the function of transmitting and optionally amplifying an opticalsignal, and (ii) an optical cladding, having the function of confiningthe optical signal in the core. For this purpose, the refractive indexesof the core (n_(c)) and of the cladding (n_(g)) are such thatn_(c)>n_(g). As is well known in the art, the propagation of an opticalsignal in a single-mode optical fiber is broken down into a fundamentalmode (known as LP01) guided in the core, and into secondary modes guidedover a certain radius in the core-cladding assembly.

Conventionally, step-index fibers, also called SMF fibers (“Single ModeFibers”) are used as line fibers for optical fiber transmission systems.These fibers exhibit a chromatic dispersion and a chromatic dispersionslope corresponding to specific telecommunication standards.

For the requirements of compatibility between the optical systems fromdifferent manufacturers, the International Telecommunication Union (ITU)has defined a recommended standard with a norm, referenced ITU-T G.652,which must be met by a Standard Single Mode Fiber (SSMF).

This G.652 standard for transmission fibers recommends inter alia, anominal range of 8.6 microns (μm) to 9.5 microns (μm) for the Mode FieldDiameter (MFD) at a wavelength of 1310 nanometers, which can vary with+/−0.4 micron (μm) due to manufacturing tolerances; a maximum of 1260nanometers for the cable cut-off wavelength; a range of 1300 nanometersto 1324 nanometers for the dispersion cancellation wavelength (denotedλ₀); and a maximum chromatic dispersion slope of 0.092 ps/(nm²·km)(i.e., ps/nm²/km).

The cable cut-off wavelength is conventionally measured as thewavelength at which the optical signal is no longer single mode afterpropagation over 22 meters of fiber, such as defined by Subcommittee 86Aof the International Electrotechnical Commission in the IEC 60793-1-44standard. In most cases, the secondary mode most resistant to bendinglosses is the LP11 mode. The cable cut-off wavelength is, therefore, thewavelength beyond which the LP11 mode is sufficiently weakened afterpropagation over 22 meters of fiber. The method proposed by the standardinvolves considering that the optical signal is single mode when theattenuation of the LP11 mode is greater than or equal to 19.3 dB.

Moreover, for a given optical fiber, a so-called MAC value is defined asthe ratio of the mode field diameter of the fiber at 1550 nanometersover the effective cut-off wavelength λ_(ceff). The cut-off wavelengthis conventionally measured as the wavelength at which the optical signalis no longer single mode after propagation over two meters of fiber, asdefined by Subcommittee 86A of the International ElectrotechnicalCommission in the IEC 60793-1-44 standard. The MAC constitutes aparameter for assessing the performances of the fiber, in particular forfinding a compromise between the mode field diameter, the effectivecut-off wavelength, and the bending losses.

Commonly assigned U.S. Patent Application Publication No. US2007/0280615(and its counterpart European Patent Application No. 1,845,399) andcommonly assigned U.S. Patent Application Publication No. US2007/0127878(and its counterpart European Patent Application No. 1,785,754) disclosea relationship between the value of the MAC at a wavelength of 1550nanometers and the bending losses at a wavelength of 1625 nanometerswith a radius of curvature of 15 millimeters in a standard step-indexfiber (SSMF). Each of these published patent applications is herebyincorporated by reference in its entirety.

Furthermore, each application establishes that the MAC influences thebending losses of the fiber and that reducing the MAC value reducesthese bending losses. Reducing the mode field diameter and/or increasingthe effective cut-off wavelength reduces the MAC value but may lead tononcompliance with the G.652 standard, making the fiber commerciallyincompatible with some transmission systems.

The reduction of the bending losses, while retaining certain opticaltransmission parameters, constitutes a challenge for Fiber-To-The-Home(FTTH) applications.

The International Telecommunications Union (ITU) has also definedrecommended standards referenced ITU-T G.657A and ITU-T G.657B, whichmust be met by the optical fibers intended for FTTH applications,particularly in terms of resistance to bending losses. The G.657Astandard imposes limits on values for bending losses but seeks, aboveall, to preserve compatibility with the G.652 standard, particularly interms of mode field diameter (MFD) and chromatic dispersion. On theother hand, the G.657B standard imposes strict bending loss limits,particularly (i) bending losses less than 0.003 dB/turn at a wavelengthof 1550 nanometers for a radius of curvature of 15 millimeters, and (ii)bending losses less than 0.01 dB/turn at a wavelength of 1625 nanometersfor a radius of curvature of 15 millimeters.

Commonly assigned U.S. Patent Application Publication No. US2007/0280615(and its counterpart European Patent Application No. 1,845,399) and U.S.Patent Application Publication No. US2007/0127878 (and its counterpartEuropean Patent Application No. 1,785,754) propose fiber profiles havinglimited bending losses, corresponding in particular to the criteria ofthe G.657A and G.657B standards. The profiles described in theseEuropean patent applications, however, make it possible to achieve onlythe bending loss limits imposed by the G.657B standard.

U.S. Pat. No. 7,164,835 and U.S. Pat. No. 7,440,663, each of which ishereby incorporated by reference in its entirety, also describe fiberprofiles exhibiting limited bending losses. The disclosed fibers,however, correspond only to the criteria of the G.657A and G.657Bstandards, particularly in terms of mode field diameter and chromaticdispersion.

For certain applications, the reduction of the bending losses isessential, especially when the fiber is intended to be stapled or coiledin a miniaturized optical box.

Hole-assisted fiber technology makes it possible to achieve excellentperformances with respect to bending losses, but this technology iscomplex and expensive to implement and cannot be used for fibersintended for low-cost FTTH systems.

A need therefore exists for an optical fiber having a resistance tobending losses that is better (e.g., an order of ten times better) thanthe limits imposed by the G.657B standard. The fiber meeting thiscriterion should also remain compatible with the G.652 standard in termsof transmission profile and, in particular, mode field diameter. Thisappreciable improvement of bending losses may be achieved to thedetriment of a higher cut-off wavelength, provided that (i) the directlyhigher order LP11 mode is sufficiently attenuated, and (ii) the lengthof fiber required for the attenuation of the LP11 mode to reach 19.3 dBat a wavelength of 1260 nanometers is less than 90 meters.

SUMMARY OF THE INVENTION

Accordingly, the present invention includes a fiber with a central core,an intermediate cladding, a depressed trench, and an external opticalcladding. The refractive index profile improves the bending losses by afactor of ten relative to the constraints imposed by the G.657Bstandard, while retaining a mode field diameter compatible with theG.652 standard and ensuring a sufficient attenuation of the LP11 mode.

In particular, the surface of the core, as well as the surface and thevolume of the depressed trench, are designed to considerably improve thebending losses. In the context of the present invention, the surface ofthe core or the surface of the depressed trench should not extendgeometrically but should correspond to values taking into account twodimensions—the product of the radius and the refractive indexdifference. Similarly, the volume of the depressed trench corresponds toa value taking into account three dimensions—the product of the squareof the radius and the refractive index difference.

The invention embraces, more particularly, a single-mode optical fiber,including, from the center toward the periphery, a central core, anintermediate cladding, a depressed trench, and an external opticalcladding. The central core has a radius r₁ and a positive refractiveindex difference Δn₁ with the external optical cladding. Theintermediate cladding has a radius r₂ and a positive refractive indexdifference Δn₂ with the external optical cladding. The refractive indexdifference Δn₂ is less than the core's refractive index difference Δn₁.The depressed trench has a radius r₃ and a negative refractive indexdifference Δn₃ with the external optical cladding.

The present optical fiber possesses (i) a mode field diameter (MFD)between about 8.6 microns (μm) and 9.5 microns (μm) at a wavelength of1310 nanometers, and (ii) bending losses less than about 0.25×10⁻³dB/turn for a radius of curvature of 15 millimeters and a wavelength of1550 nanometers. The length of fiber required for the attenuation of theLP11 mode to reach 19.3 dB at a wavelength of 1260 nanometers is lessthan about 90 meters (e.g., less than about 60 meters, such as less thanabout 40 meters).

According to one fiber embodiment, the surface integral of the centralcore (V₀₁), defined as

V₀₁ = ∫₀^(r 1)Δ n(r)⋅ r ≈ r₁ × Δ n₁,

is between about 20.0×10⁻³ micron and 23.0×10⁻³ micron.

The surface integral of the depressed trench (V03), defined as

V₀₃ = ∫_(r 2)^(r 3)Δ n(r)⋅ r ≈ (r₃ − r₂) × Δ n₃,

is between about −55.0×10⁻³ micron and −30.0×10⁻³ micron.

The volume integral of the depressed trench (V₁₃), defined as,

V₁₃ = 2 ⋅ ∫_(r 2)^(r 3)Δ n(r)⋅ r ⋅ r ≈ (r₃² − r₂²) × Δ n₃,

is between about −1200×10⁻³ μm² and −750×10⁻³ μm².

In exemplary embodiments, the fiber has physical properties andoperational parameters with improved resistance to bending losses. Forinstance, the fiber has an effective cut-off wavelength λ_(ceff) greaterthan about 1350 nanometers, the effective cut-off wavelength beingmeasured as the wavelength at which the optical signal becomes singlemode after propagation over two meters of fiber. The fiber has, for awavelength of 1550 nanometers, bending losses less than or equal toabout 7.5×10⁻³ dB/turn for a radius of curvature of 10 millimeters,bending losses less than or equal to about 0.05 dB/turn for a radius ofcurvature of 7.5 millimeters, and bending losses less than about 0.15dB/turn for a radius of curvature of 5 millimeters.

The fiber disclosed herein also shows reduced bending losses at higherwavelengths. For example, at a wavelength of 1625 nanometers, the fiberhas bending losses less than about 1.5×10⁻³ dB/turn for a radius ofcurvature of 15 millimeters, bending losses less than or equal to about25×10⁻³ dB/turn for a radius of curvature of 10 millimeters, bendinglosses less than or equal to about 0.08 dB/turn for a radius ofcurvature of 7.5 millimeters, and bending losses less than about 0.25dB/turn for a radius of curvature of 5 millimeters.

Accordingly, in an exemplary embodiment, the fiber has a cut-offwavelength between about 1300 nanometers and 1400 nanometers, with thecut-off wavelength measured as the wavelength at which the opticalsignal is no longer multimode after propagation over five meters offiber. Cut-off wavelength is distinguished from cable cut-off, measuredas the wavelength at which the attenuation of the LP11 mode is greaterthan or equal to 19.3 dB after propagation over 22 meters of fiber. Thefiber has a cable cut-off wavelength between about 1250 nanometers and1300 nanometers.

Another definition at issue here is the theoretical cut-off wavelengthdefined as the wavelength from which the LP11 mode is propagated inleaky mode. In one embodiment, the fiber has a theoretical cut-offwavelength less than or equal to about 1250 nanometers. The fiber has anattenuation of the LP11 mode greater than about 5 dB (e.g., about 15 dbor more) after propagation over 22 meters of fiber at a wavelength of1260 nanometers.

The operational parameters described previously result from exemplaryphysical properties of the fiber. In one embodiment, the central core ofthe fiber has a radius between about 3.8 microns and 4.35 microns; theintermediate cladding has a radius between about 8.5 microns and 9.7microns; and the depressed trench has a radius between about 13.5microns and 16 microns, which can be less than or equal to 15 microns(e.g., between about 13.5 and 15 microns). The central core typicallyhas a refractive index difference (Δn₁) with the external opticalcladding between about 5.3×10⁻³ and 5.7×10⁻³.

As noted, the refractive index profile of a fiber is plotted in terms ofthe difference between refractive index values at points on the radiusof the fiber and the external optical cladding. The intermediatecladding has a refractive index difference with the optical claddingbetween about 0.1×10⁻³ and 0.6×10⁻³. The depressed trench has arefractive index difference with the optical cladding between about−10.0×10⁻³ and −5.0×10⁻³. The fiber has a zero chromatic dispersionwavelength between about 1300 nanometers and 1324 nanometers, and achromatic dispersion slope value at the zero chromatic dispersionwavelength of less than about 0.092 ps/(nm²·km).

The present invention also relates to an optical box receiving at leastone portion of optical fiber disclosed herein. In such an optical box,the fiber can be arranged with a radius of curvature less than about 15millimeters, which can be about 5 millimeters. The present inventionalso relates to an optical fiber system to the subscriber's home (FTTH)including at least one portion of optical fiber disclosed herein.

The foregoing, as well as other characteristics and advantages of thepresent invention, and the manner in which the same are accomplished,are further specified within the following detailed description and itsaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross section of a single-mode fiber with claddinglayers at respective radii extending from the center.

FIG. 2 depicts the nominal refractive index profile of the exemplarysingle-mode fiber of FIG. 1 in accordance with the present invention.

DETAILED DESCRIPTION

As depicted in FIG. 1, the optical fiber (10) of the invention has acentral core (11), an intermediate cladding (12), and a depressedcladding (13). For purposes herein and without limiting the scope of theinvention, depressed cladding means a radial portion of the fiber (10)having a refractive index less than the refractive index of the externaloptical cladding (14). Typically, the central core (11), theintermediate cladding (12), and the depressed cladding (13) are obtainedby chemical vapor deposition in a silica tube. The external opticalcladding (14) includes the silica tube and the overcladding on the tube.In typical embodiments, the overcladding is generally natural or dopedsilica, but can also be obtained by any other deposition technique(e.g., vapor axial deposition (“VAD”) or outside vapor deposition(“OVD”)).

FIG. 2 illustrates a refractive index profile for the optical fiber (10)of FIG. 1. The profile of FIG. 2 is a set profile (i.e., representativeof the theoretical profile of the fiber), but the optical fiber actuallyobtained after fiber drawing of a preform may have a slightly differentprofile.

In a manner known in the art, an optical fiber (10) is obtained bypreform drawing. By way of example, the preform may be a veryhigh-quality glass tube (e.g., pure silica), which eventually forms partof the external optical cladding (14). The external optical cladding(14) surrounds the central core (11) and the internal claddings (12, 13)of the fiber (10). This tube can then be overcladded to increase itsdiameter before going through the fiber-drawing operation on afiber-drawing tower. For the production of the preform, the tube isgenerally mounted horizontally and held at both ends by glass bars on alathe. Then, the tube is rotated and heated locally for the depositionprocess that determines the composition of the preform, which, in turn,determines the optical characteristics of the drawn fiber.

The fiber includes a central core (11) having a refractive indexdifference Δn₁ with the external cladding (14), which functions as anoptical cladding. The fiber (10) further includes an intermediatecladding (12) having a refractive index difference Δn₂ with the externaloptical cladding (14) and a depressed trench cladding (13) having arefractive index difference Δn₃ with the external optical cladding (14).The refractive indexes in the central core (11), the intermediatecladding (12), and the depressed trench (13) are substantially constantthroughout their respective widths, as depicted in FIG. 2. FIG. 1illustrates that the width of the core (11) is defined by its radius r₁and the width of the claddings by their respective external radii, r₂and r₃. The external optical cladding is denoted as r₄.

In order to define a set refractive index profile for an optical fiber,the refractive index value of the external optical cladding (14) isgenerally taken as a reference (n_(g)). The refractive index values ofthe central core (11), the intermediate cladding (12), and the depressedtrench cladding (13) are then presented in FIG. 2 as index differencesΔn_(1,2,3), respectively. Generally, the external optical cladding (14)is composed of silica, but this cladding can be doped to increase orreduce its refractive index (e.g., to modify the propagationcharacteristics of the signal).

Each fiber profile section shown in FIG. 2 (21-24) can also be definedon the basis of integrals that link the index variations with the radiusof each section of the fiber (10). It is thus possible to define threesurface integrals for the fiber (10) of the invention, representative ofthe surface of the core V₀₁, the surface of the intermediate claddingV₀₂, and the surface of the depressed trench V₀₃. The expression“surface” should not be understood geometrically but rather ascorresponding to a value taking two dimensions into account. These threesurface integrals can be expressed as follows:

V₀₁ = ∫₀^(r 1)Δ n(r)⋅ r ≈ r₁ × Δ n₁V₀₂ = ∫_(r 1)^(r 2)Δ n(r)⋅ r ≈ (r₂ − r₁) × Δ n₂V₀₃ = ∫_(r 2)^(r 3)Δ n(r)⋅ r ≈ (r₃ − r₂) × Δ n₃.

Similarly, it is possible to define three volume integrals for theoptical fiber (10) of the invention, representative of the volume of thecore V₁₁, the volume of the intermediate cladding V₁₂, and the volume ofthe depressed trench V₁₃. The expression “volume” should not beunderstood geometrically but rather as corresponding to a value takingthree dimensions into account. These three volume integrals can beexpressed as follows:

V₁₁ = 2 ⋅ ∫₀^(r 1)Δ n(r)⋅ r ⋅ r ≈ r₁² × Δ n₁V₁₂ = 2 ⋅ ∫_(r 1)^(r 2)Δ n(r)⋅ r ⋅ r ≈ (r₂² − r₁²) × Δ n₂V₁₃ = 2 ⋅ ∫_(r 2)^(r 3)Δ n(r)⋅ r ⋅ r ≈ (r₃² − r₂²) × Δ n₃.

Unless otherwise noted, the examples presented in the following TablesI-IV are predictive simulations. In this regard, Table I (below) shows30 prophetic examples of fiber profiles according to exemplaryembodiments of the invention in comparison with three SSMF fiberprofiles and one fiber profile corresponding to the G.657A and G.657Bstandards (noted as “BIF” for Bend Insensitive Fiber). Draka Comteqmarkets a bend insensitive fiber having a good resistance to bendinglosses under the trademark BendBright^(XS)®. The values in the tablescorrespond to the set profiles for each fiber.

The first column of Table I assigns a reference to each example; thenext three columns give the values of the radii of the core (11), theintermediate cladding (12), and the depressed trench (13), respectively.The next three columns give the corresponding values of the refractiveindex differences with the external optical cladding (14). Therefractive index values are measured at a wavelength of 633 nanometers.Table I also shows the surface integral and volume integral values ofthe core (11), the intermediate cladding (12), and the depressed trench(13), as defined previously.

TABLE I V₀₁ V₀₂ V₀₃ V₁₁ V₁₂ V₁₃ r₁ r₂ r₃ Δn₁ Δn₂ Δn₃ (μm) (μm) (μm)(μm²) (μm²) (μm²) (μm) (μm) (μm) [10⁻³] [10⁻³] [10⁻³] [10⁻³] [10⁻³][10⁻³] [10⁻³] [10⁻³] [10⁻³] BIF 3.93 9.38 14.72 5.26 0.13 −5.01 20.7 0.7−26.8 81.1 9.4 −645 SSMF1 4.35 13.92 5.00 −0.20 21.8 −1.9 0.0 94.6 −35.00 SSMF2 4.51 13.92 5.00 −0.20 22.5 −1.9 0.0 101.5 −34.7 0 SSMF3 4.5513.92 5.24 −0.20 23.8 −1.9 0.0 108.4 −34.6 0 Ex 1 3.97 9.38 14.25 5.560.11 −9.74 22.1 0.6 −47.4 87.7 8.3 −1120 Ex 2 3.98 8.65 13.83 5.52 0.21−9.56 22.0 1.0 −49.5 87.3 12.6 −1113 Ex 3 4.01 8.95 14.39 5.38 0.20−9.27 21.6 1.0 −50.4 86.5 13.1 −1177 Ex 4 3.98 8.77 13.79 5.56 0.33−9.25 22.1 1.6 −46.5 87.9 19.9 −1049 Ex 5 3.90 8.70 14.31 5.58 0.44−7.93 21.8 2.1 −44.5 84.8 26.6 −1024 Ex 6 4.03 9.17 14.04 5.45 0.21−9.62 21.9 1.1 −46.8 88.3 14.0 −1087 Ex 7 4.04 8.61 14.39 5.56 0.15−7.05 22.4 0.7 −40.7 90.5 8.7 −937 Ex 8 3.83 8.94 13.92 5.69 0.52 −8.5121.8 2.6 −42.4 83.7 33.8 −969 Ex 9 4.01 8.97 14.39 5.38 0.39 −8.45 21.61.9 −45.8 86.4 25.1 −1071 Ex 10 3.84 9.30 14.38 5.49 0.48 −9.38 21.1 2.6−47.7 81.0 34.2 −1129 Ex 11 3.82 9.01 13.55 5.67 0.57 −9.63 21.7 2.9−43.7 82.8 37.7 −986 Ex 12 4.03 8.84 14.28 5.30 0.11 −9.52 21.3 0.5−51.8 85.9 6.6 −1197 Ex 13 3.96 8.61 13.86 5.58 0.31 −7.87 22.1 1.4−41.3 87.6 17.9 −928 Ex 14 3.92 8.78 13.84 5.55 0.32 −8.75 21.7 1.5−44.3 85.2 19.7 −1002 Ex 15 3.88 9.09 14.35 5.62 0.34 −7.84 21.8 1.8−41.2 84.5 23.1 −965 Ex 16 4.02 9.65 14.35 5.37 0.14 −9.72 21.6 0.8−45.7 86.7 10.6 −1097 Ex 17 4.01 9.19 14.39 5.32 0.36 −8.74 21.3 1.9−45.4 85.6 24.9 −1072 Ex 18 3.93 9.30 14.48 5.30 0.51 −7.76 20.8 2.7−40.1 81.7 36.0 −955 Ex 19 3.93 9.26 13.53 5.34 0.51 −9.74 21.0 2.7−41.6 82.3 36.0 −949 Ex 20 3.93 9.25 13.53 5.31 0.50 −9.93 20.8 2.7−42.5 81.9 35.3 −967 Ex 21 3.93 9.28 14.47 5.31 0.53 −7.51 20.9 2.8−39.0 82.0 37.5 −926 Ex 22 3.93 8.50 15.00 5.48 0.50 −5.00 21.5 2.3−32.5 84.6 28.4 −764 Ex 23 3.93 9.25 13.65 5.37 0.50 −9.90 21.1 2.7−43.5 83.0 35.1 −997 Ex 24 3.93 8.50 15.50 5.33 0.51 −5.00 21.0 2.3−35.0 82.4 28.8 −840 Ex 25 3.93 9.27 13.65 5.31 0.52 −9.80 20.9 2.8−42.9 82.1 36.9 −983 Ex 26 3.94 8.50 15.00 5.43 0.50 −5.00 21.4 2.3−32.5 84.3 28.6 −764 Ex 27 3.94 9.25 13.54 5.30 0.56 −9.87 20.9 3.0−42.3 82.3 39.2 −964 Ex 28 3.94 9.26 13.50 5.33 0.51 −9.88 21.0 2.7−41.9 82.8 35.5 −954 Ex 29 3.95 9.29 13.91 5.30 0.50 −8.93 20.9 2.7−41.2 82.6 35.4 −957 Ex 30 3.93 8.50 15.50 5.32 0.57 −5.00 20.9 2.6−35.0 82.1 32.2 −840

The fiber (10) according to the invention is a step-index fibercomprising a central core (11), an intermediate cladding (12), and adepressed trench (13). It is noted from Table I that the central core(11) has a radius r₁ between 3.8 microns and 4.35 microns, typicallybetween 3.8 microns and 4.05 microns (i.e., narrower than the core of anSSMF fiber). The fiber (10) has a refractive index difference Δn₁ (21)with the external optical cladding (14) between 5.3×10⁻³ and 5.7×10⁻³(i.e., greater than an SSMF fiber). The surface integral of the core V₁₀is between 20.0×10⁻³ micron and 23.0×10⁻³ micron, and the volumeintegral of the core V₁₁ is between 81×10⁻³ μm² and 91×10⁻³ μm².

It is also noted from Table I that the fiber according to the inventionhas a depressed trench (13). The depressed trench (13) has a largevolume and makes it possible to greatly limit bending losses. Table Ithus shows that the depressed trench (13) has a radius r₃ between 13.5microns and 16 microns and a refractive index difference Δn₃ (23) withthe external optical cladding (14) between −10.0×10⁻³ and −5.0×10⁻³.Table I also shows that the surface integral of the depressed trenchV₀₃, as defined previously, is between −55.0×10⁻³ micron and −30.0×10⁻³micron, and the volume integral of the depressed trench V₁₃, as definedpreviously, is between −1200×10⁻³ μm² and −750×10⁻³ μm².

According to an exemplary embodiment, the radius of the depressedcladding r₃ can be limited to 15 microns to further reduce the cost offiber production. (Note that only Example 24 and Example 30 have adepressed cladding with a radius greater than 15 microns.) In fact, thedepressed trench (13) can be produced by plasma chemical vapordeposition (PCVD), making it possible to incorporate a large quantity offluorine in the silica to form deeply depressed claddings. The part ofthe fiber (10) corresponding to the tube and to the PCVD deposition is,however, the most expensive, so it is therefore sought to limit thispart as much as possible. It is also possible to envisage producing thedepressed trench (13) by incorporation of micro-holes or micro-bubblesrather than by fluorine doping. Fluorine doping, however, remains easierto control for industrial production than the incorporation ofmicro-bubbles.

A depressed trench (13) corresponding to the surface and volume criteriadefined previously makes it possible to achieve a good compromisebetween greatly reduced bending losses relative to the existing fibersand a sufficiently consistent leakage regime of the LP11 mode at awavelength of 1260 nanometers.

As is clear from Table IV, which is discussed in detail hereafter, thefiber according to the invention has bending losses which are ten times(10×) smaller than the limits imposed by the G.657B standard. On theother hand, the fiber according to the invention does not strictlycomply with the G.657 standard in terms of cut-off wavelength. As isclear from Table III, which is also discussed in detail hereafter, thefiber according to the invention has an effective cut-off wavelengthλ_(ceff) greater than 1350 nanometers and a cable cut-off wavelengthλ_(cc) between 1250 nanometers and 1300 nanometers. Nevertheless, thefiber disclosed herein ensures that the higher order LP11 modes arepropagated in the 1260 nanometers leaky mode regime.

It is also noted from Table I that an exemplary embodiment of the fiberhas an intermediate cladding (12) between the central core (11) and thedepressed trench (13). This intermediate cladding (12) makes it possibleto limit the effects of the depressed trench (13) on the propagation ofthe optical signal in the core. Table I shows that the intermediatecladding (12) has a radius r₂ between 8.5 microns and 9.7 microns and arefractive index difference Δn₂ (22) with the optical cladding between0.1×10⁻³ and 0.6×10⁻³. Table I shows that the surface integral of theintermediate cladding V₀₂, as defined previously, is between 0.5×10⁻³micron and 3.0×10⁻³ micron. The volume integral of the intermediatecladding V₁₂, as defined previously, is between 6×10⁻³ μm² and 40×10⁻³μm².

The central core (11) of a fiber (10) according to the invention isdesigned, in combination with the intermediate cladding (12), toguarantee parameters of optical transmission in the fiber in conformitywith the G.652 and G.657A standards, particularly in terms of mode fielddiameter and chromatic dispersion. This also helps ensure compatibilitywith fibers of other optical systems.

Table II (below) shows the optical transmission characteristics forfibers according to the invention. The first column repeats thereferences of Table I. The following columns provide, for each fiberprofile, the mode field diameter (MFD) values for wavelengths of 1310nanometers and 1550 nanometers, zero dispersion wavelength (ZDW), andzero dispersion slope (ZDS).

TABLE II MFD1310 MFD1550 ZDW ZDS (μm) (μm) (nm) ps/(nm² · km) BIF 8.809.90 1320 0.0878 SSMF1 9.14 10.31 1314 0.0855 SSMF2 9.27 10.39 13090.0871 SSMF3 9.18 10.25 1306 0.088 Ex1 8.63 9.62 1314 0.0899 Ex2 8.649.56 1308 0.0924 Ex3 8.76 9.71 1310 0.0918 Ex4 8.69 9.63 1309 0.0921 Ex58.68 9.66 1313 0.0914 Ex6 8.76 9.73 1310 0.0913 Ex7 8.66 9.58 13070.0916 Ex8 8.64 9.65 1317 0.0904 Ex9 8.86 9.84 1311 0.0918 Ex10 8.769.81 1319 0.0901 Ex11 8.67 9.68 1317 0.0908 Ex12 8.75 9.69 1308 0.0923Ex13 8.65 9.59 1310 0.0917 Ex14 8.66 9.62 1312 0.0914 Ex15 8.64 9.651317 0.0897 Ex16 8.79 9.81 1314 0.0898 Ex17 8.89 9.90 1312 0.0913 Ex188.95 10.01 1317 0.0905 Ex19 8.91 9.94 1315 0.0913 Ex20 8.92 9.95 13150.0914 Ex21 8.96 10.02 1317 0.0905 Ex22 8.80 9.81 1314 0.0906 Ex23 8.899.91 1315 0.0913 Ex24 8.88 9.91 1314 0.0909 Ex25 8.94 9.97 1315 0.0914Ex26 8.83 9.84 1313 0.0908 Ex27 8.97 10.00 1314 0.0917 Ex28 8.93 9.951314 0.0915 Ex29 8.95 9.99 1315 0.0911 Ex30 8.92 9.95 1314 0.0911

It is noted from Table II that the fiber (10) according to the inventionis compatible with fibers corresponding to the criteria of the G.652standard. In particular, the fiber disclosed herein has a mode fielddiameter MFD in the standardized range of values from 8.6 microns to 9.5microns at 1310 nanometers, a zero dispersion wavelength between 1300nanometers and 1324 nanometers, and a zero dispersion slope of less than0.092 ps/(nm²·km). Each of these values is in accordance with the G.652standard.

On the other hand, as shown by Table III (below), the fiber has aneffective cut-off wavelength λ_(ceff) greater than 1350 nanometers. Asdiscussed, the cut-off wavelength is measured as being the wavelength atwhich the optical signal is no longer single mode after propagation overtwo meters of fiber, as defined by Subcommittee 86A of the InternationalElectrotechnical Commission in the IEC 60793-1-44 standard.

This increased effective cut-off wavelength value leads to a cablecut-off wavelength value λ_(cc) between 1250 nanometers and 1300nanometers. The cable cut off wavelength is measured as the wavelengthat which the optical signal is no longer single mode after propagationover 22 meters of fiber, as defined by Subcommittee 86A of theInternational Electrotechnical Commission in the IEC 60793-1-44standard. The optical signal is single mode when the attenuation of theLP11 mode is greater than or equal to 19.3 dB. The G.652 and G.657standards both impose a maximum value of 1260 nanometers for the cablecut-off wavelength.

One purpose of the developments disclosed herein is to produce fibersthat can be used on all of the transmission bandwidths exploited byoptical systems (i.e., fibers that can be used in single modepropagation from the original bandwidth (OB), which extends from 1260nanometers to 1360 nanometers and as far as the ultra-long (UL)bandwidth beyond 1625 nanometers). A low effective cut-off wavelengthmakes it possible to guarantee the possibility of using the fiber acrossall of the available bandwidths.

The simulations of Table III (below), however, show that the directlyhigher order LP11 mode is propagated according to a leaky mode from awavelength of 1260 nanometers. The fiber disclosed herein therefore canbe used in single-mode transmission over the original bandwidth (OB:1260 nanometers to 1360 nanometers).

Table III (below) shows several cut-off wavelength values for opticalfibers according to the invention. The first column of Table III repeatsthe references of Table I.

The column “Theoretical Fiber Cutoff” provides a theoretical cut-offwavelength value, which corresponds to the transition wavelength betweena guided propagation of the LP11 mode and a propagation in leaky mode ofthis LP11 mode. For working wavelengths beyond this effective cut-offwavelength, the LP11 mode is propagated in leaky mode.

The column “Standard Fiber Cutoff” corresponds to the effective cut-offwavelength λ_(ceff) as defined by Subcommittee 86A of the InternationalElectrotechnical Commission in the IEC 60793-1-44 standard.

The column “5-m Fiber Cutoff” corresponds to the cut-off wavelengthmeasured as the wavelength at which the optical signal is no longermultimode after propagation over five meters of fiber. This valuetherefore corresponds to the effective cut-off wavelength measured afterpropagation over five meters of fiber instead of two meters of fiber.

The column “Standard Cable Cutoff” corresponds to the cable cut-offwavelength λ_(cc) as defined by Subcommittee 86A of the InternationalElectrotechnical Commission in the IEC 60793-1-44 standard. According tothe recommendation of Subcommittee 86A of the InternationalElectrotechnical Commission in the IEC 60793-1-44 standard, the cablecut-off wavelength λ_(cc) is determined by positioning the fiber intotwo 40-millimeter radius loops and by arranging the remainder of thefiber (i.e., 21.5 meters of fiber) on a mandrel with a radius of 140millimeters.

The column “Straight Cable Cutoff” corresponds to the cable cut-offwavelength by positioning the fiber into two loops, each having a40-millimeter radius, and by arranging the remainder of the fiber (i.e.,21.5 meters of fiber) virtually straight.

The column “LP11 LL@1260 after 22 m” indicates the leakage losses of theLP11 mode after propagation over 22 meters of virtually straight fiber.

The column “Length-19.3 dB LP11 LL@1260 nm” indicates the length offiber required to achieve leakage losses of the LP11 mode equal to 19.3dB with the fiber being kept virtually straight. This indicates at whichdistance the fiber, arranged virtually straight, is single mode withinthe meaning of the G.652 and G.657 standards.

TABLE III LP11 LL Length- Std 5-m Std Straight @1260 nm 19.3 dB FiberCutoff Fiber Fiber Cable Cable after LP11 LL (theory) Cutoff CutoffCutoff Cutoff 22 m @1260 nm (nm) (nm) (nm) (nm) (nm) (dB) (m) BIF 11971270 1234 1196 1208 180 2 SSMF1 1287 1226 1226 1151 1151 2 212 SSMF21334 1267 1267 1188 1188 0 >1000 SSMF3 1381 1311 1311 1231 1231 0 >1000Ex 1 1235 1437 1366 1290 1284 9 48 Ex 2 1231 1438 1368 1287 1284 9 45 Ex3 1228 1466 1392 1297 1301 7 61 Ex 4 1250 1420 1354 1290 1283 6 69 Ex 51243 1419 1353 1287 1280 10 44 Ex 6 1246 1430 1361 1292 1285 8 56 Ex 71248 1403 1343 1284 1278 8 52 Ex 8 1249 1386 1326 1274 1270 11 40 Ex 91250 1436 1367 1297 1291 5 89 Ex 10 1233 1435 1362 1287 1280 10 42 Ex 111250 1379 1321 1271 1268 10 41 Ex 12 1213 1467 1393 1300 1298 9 48 Ex 131243 1383 1323 1271 1266 16 27 Ex 14 1232 1397 1333 1271 1265 16 26 Ex15 1239 1392 1331 1272 1267 15 28 Ex 16 1234 1424 1354 1283 1277 11 39Ex 17 1244 1429 1360 1291 1284 9 49 Ex 18 1242 1382 1322 1268 1264 18 24Ex 19 1243 1360 1304 1257 1258 26 16 Ex 20 1238 1362 1305 1256 1255 2417 Ex 21 1247 1376 1319 1267 1266 15 28 Ex 22 1249 1351 1302 1259 126218 23 Ex 23 1246 1378 1319 1268 1264 17 25 Ex 24 1235 1373 1317 12641260 18 24 Ex 25 1243 1371 1313 1263 1260 22 20 Ex 26 1247 1350 13001257 1260 22 19 Ex 27 1248 1367 1310 1263 1263 17 25 Ex 28 1245 13621306 1259 1259 24 18 Ex 29 1244 1371 1314 1264 1260 20 21 Ex 30 12401375 1319 1267 1263 17 24

It is noted from Table III that the standard effective cut-offwavelength λ_(ceff) (i.e., as measured according to the recommendationsof Subcommittee 86A of the International Electrotechnical Commission inthe IEC 60793-1-44 standard) is greater than 1350 nanometers. Similarly,it is noted from Table III that the standard cable cut-off wavelengthλ_(cc) (i.e., as measured according to the recommendations ofSubcommittee 86A of the International Electrotechnical Commission in theIEC 60793-44 standard) is between 1250 nanometers and 1300 nanometers(i.e., often greater than the limit of 1260 nanometers imposed by theG.652 and G.657 standards).

It is noted, however, from Table III that the LP11 mode is neverthelesshighly attenuated from a wavelength of 1260 nanometers. In fact, the“theoretical” fiber cut-off wavelength is less than or equal to 1250nanometers. Thus, the higher order LP11 mode is propagated in a leakymode regime in the original bandwidth, and only the fundamental moderemains guided in the fiber of the invention from a wavelength of 1260nanometers.

Similarly, it is noted from Table III that the fiber cut-off wavelengthis significantly reduced after only five meters of propagation in thefiber. Thus, the cut-off wavelength, measured as the wavelength at whichthe optical signal is no longer single mode after propagation over fivemeters of fiber, is between 1300 nanometers and 1400 nanometers for afiber according to the invention.

Moreover, Table III clearly shows that the LP11 mode is already wellattenuated after 22 meters of propagation. It is noted in particularthat the attenuation of the LP11 mode in a fiber (10) according to theinvention is greater than the attenuation of the LP11 mode in an SSMFfiber when the fiber is arranged virtually straight. In fact, in an SSMFfiber, it is the bends that make it possible to highly attenuate theLP11 mode. Thus, the fiber has an attenuation of the LP11 mode greaterthan 5 dB after 22 meters of propagation in straight fiber at awavelength of 1260 nanometers (e.g., LP11-mode attenuation of about 10dB or more).

Although Table III shows that attenuation of at least 19.3 dB of theLP11 mode is not always reached within 22 meters, such LP11-modeattenuation is achieved relatively rapidly, typically within 90 meters(e.g., about 50 meters or less).

Thus, the failure to comply in the strictest sense with the G.652 andG.657 standards in terms of cut-off wavelength is minimized by the factthat the higher order LP11 mode is sufficiently attenuated from awavelength of 1260 nanometers so as not to impair the quality of thepropagation of the fundamental mode.

Moreover, the increase in the effective cut-off wavelength makes itpossible to increase the value of the MAC as defined previously and,consequently, to reduce the bending losses.

Table IV (below) reports bending loss values for exemplary embodimentsof fibers as disclosed herein. The first column of Table IV repeats thereferences of Table I. The next four columns show bending loss valuesPPC for respective radii of curvature of 15 millimeters, 10 millimeters,7.5 millimeters, and 5 millimeters at a wavelength of 1550 nanometers.The next four columns give bending loss values PPC for respective radiiof curvature of 15 millimeters, 10 millimeters, 7.5 millimeters, and 5millimeters at a wavelength of 1625 nanometers.

The last column has a factor of merit (FOM) representing the order ofmagnitude of the improvement in the bending losses by the fibersaccording to the invention relative to the limits imposed by the G.657Bstandard. The FOM of Table IV is thus defined as an average of theratios between the upper limits imposed by the G.657B standard and thebending losses in the fibers of the invention for each radius ofcurvature measured.

Table IV reports on the first line the bending loss limit values imposedby the G.657B standard for each radius of curvature and for thewavelengths of 1550 nanometers and 1625 nanometers.

TABLE IV R = 15 mm R = 10 mm R = 7.5 mm R = 5 mm R = 15 mm R = 10 mm R =7.5 mm R = 5 mm FOM PPC @1550 nm (dB/turn) PPC @1625 nm (dB/turn) G.657B0.003 0.1 0.5 0.01 0.2 1 1.00 BIF 1.3E−03 2.9E−02 1.0E−01 3.3E−017.0E−03 8.4E−02 2.3E−01 6.3E−01 0.70 SSMF1 1.5E−02 6.0E−01 3.4E+001.7E+01 7.5E−02 1.7E+00 6.9E+00 2.7E+01 8.44 SSMF2 6.3E−03 3.6E−012.4E+00 1.4E+01 3.4E−02 1.0E+00 5.0E+00 2.3E+01 5.21 SSMF3 9.6E−041.1E−01 1.0E+00 8.9E+00 6.5E−03 3.6E−01 2.5E+00 1.4E+01 2.45 Ex 14.3E−05 2.0E−03 9.7E−03 3.6E−02 3.3E−04 7.3E−03 2.5E−02 7.0E−02 0.04 Ex2 4.4E−05 2.0E−03 9.2E−03 3.5E−02 3.4E−04 7.2E−03 2.4E−02 7.1E−02 0.04Ex 3 6.4E−05 2.2E−03 9.0E−03 3.2E−02 4.4E−04 7.6E−03 2.3E−02 6.4E−020.04 Ex 4 3.6E−05 2.0E−03 1.1E−02 4.5E−02 2.9E−04 7.6E−03 2.8E−028.8E−02 0.04 Ex 5 4.7E−05 2.4E−03 1.2E−02 4.6E−02 3.6E−04 8.6E−033.1E−02 9.2E−02 0.04 Ex 6 5.3E−05 2.4E−03 1.2E−02 4.4E−02 3.9E−048.6E−03 3.0E−02 8.4E−02 0.04 Ex 7 4.2E−05 2.4E−03 1.3E−02 5.1E−023.4E−04 8.9E−03 3.3E−02 1.0E−01 0.04 Ex 8 4.5E−05 2.6E−03 1.5E−026.3E−02 3.6E−04 9.9E−03 3.8E−02 1.2E−01 0.05 Ex 9 6.9E−05 2.8E−031.3E−02 4.8E−02 4.8E−04 9.7E−03 3.2E−02 9.1E−02 0.05 Ex 10 8.3E−053.0E−03 1.3E−02 4.7E−02 5.6E−04 1.0E−02 3.2E−02 8.8E−02 0.06 Ex 114.9E−05 2.9E−03 1.6E−02 7.1E−02 3.9E−04 1.1E−02 4.2E−02 1.3E−01 0.05 Ex12 9.1E−05 2.6E−03 9.5E−03 3.0E−02 6.1E−04 8.6E−03 2.3E−02 6.1E−02 0.06Ex 13 5.4E−05 2.9E−03 1.6E−02 6.5E−02 4.3E−04 1.1E−02 4.1E−02 1.3E−010.05 Ex 14 6.6E−05 3.0E−03 1.5E−02 5.6E−02 5.0E−04 1.1E−02 3.8E−021.1E−01 0.05 Ex 15 6.2E−05 3.1E−03 1.5E−02 6.3E−02 4.7E−04 1.1E−023.9E−02 1.2E−01 0.06 Ex 16 9.8E−05 3.5E−03 1.4E−02 5.3E−02 6.5E−041.2E−02 3.5E−02 1.0E−01 0.07 Ex 17 1.0E−04 3.6E−03 1.5E−02 5.6E−026.7E−04 1.2E−02 3.7E−02 1.0E−01 0.07 Ex 18 2.2E−04 6.9E−03 2.7E−021.0E−01 1.3E−03 2.1E−02 6.4E−02 1.8E−01 0.13 Ex 19 2.0E−04 7.1E−033.1E−02 1.1E−01 1.2E−03 2.3E−02 7.2E−02 2.1E−01 0.12 Ex 20 2.2E−047.4E−03 3.1E−02 1.1E−01 1.4E−03 2.4E−02 7.2E−02 2.1E−01 0.14 Ex 212.1E−04 7.1E−03 2.9E−02 1.1E−01 1.3E−03 2.2E−02 6.9E−02 2.0E−01 0.13 Ex22 1.4E−04 6.5E−03 3.1E−02 1.3E−01 1.0E−03 2.2E−02 7.7E−02 2.4E−01 0.11Ex 23 1.4E−04 5.4E−03 2.4E−02 9.0E−02 9.2E−04 1.8E−02 5.8E−02 1.7E−010.09 Ex 24 2.3E−04 7.3E−03 2.8E−02 1.0E−01 1.4E−03 2.3E−02 6.8E−022.0E−01 0.14 Ex 25 2.0E−04 6.8E−03 2.9E−02 1.0E−01 1.2E−03 2.2E−026.8E−02 2.0E−01 0.12 Ex 26 1.7E−04 7.4E−03 3.4E−02 1.3E−01 1.2E−032.4E−02 8.2E−02 2.5E−01 0.12 Ex 27 2.0E−04 7.1E−03 3.0E−02 1.1E−011.2E−03 2.3E−02 7.1E−02 2.1E−01 0.12 Ex 28 1.9E−04 7.0E−03 3.0E−021.1E−01 1.2E−03 2.3E−02 7.2E−02 2.1E−01 0.12 Ex 29 2.0E−04 7.0E−032.9E−02 1.0E−01 1.3E−03 2.2E−02 6.8E−02 2.0E−01 0.13 Ex 30 2.3E−047.4E−03 2.9E−02 1.1E−01 1.4E−03 2.3E−02 7.0E−02 2.1E−01 0.14

It is noted from Table IV that the bending losses of the fiberscorresponding to the profile according to the invention are clearly lessthan the limits imposed by the G.657B standard.

Thus, the disclosed optical fiber has, for a wavelength of 1550nanometers, bending losses less than 0.25×10⁻³ dB/turn for a radius ofcurvature of 15 millimeters, as compared to a limit of 3×10⁻³ dB/turnimposed by the G.657B standard. The fiber further has bending lossesless than or equal to 7.5×10⁻³ dB/turn for a radius of curvature of 10millimeters, as compared against a limit of 0.1 dB/turn imposed by theG.657B standard. The bending losses are less than or equal to 0.05dB/turn for a radius of curvature of 7.5 millimeters, as against a limitof 0.5 dB/turn imposed by the G.657B standard, and bending losses lessthan 0.15 dB/turn for a radius of curvature of 5 millimeters.

The bending losses at a wavelength of 1550 nanometers in a fiberaccording to the invention have been improved by a factor greater than10× relative to the limits of the G.657B standard.

Similarly, the fiber according to the invention exhibits, for awavelength of 1625 nanometers, bending losses less than 1.5×10⁻³ dB/turnfor a radius of curvature of 15 millimeters, as compared to a limit of10×10⁻³ dB/turn imposed by the G.657B standard. The bending losses areless than or equal to 25×10⁻³ dB/turn for a radius of curvature of 10millimeters, as compared to a limit of 0.2 dB/turn imposed by the G.657Bstandard. The fiber exhibits bending losses less than or equal to 0.08dB/turn for a radius of curvature of 7.5 millimeters, as against a limitof 1 dB/turn imposed by the G.657B standard, and bending losses lessthan 0.25 dB/turn for a radius of curvature of 5 millimeters.

The bending losses at a wavelength of 1625 nanometers in a fiberaccording to the invention have been improved by a factor of 10 relativeto the limits of the G.657B standard. It should be noted that, withinthe framework of an industrial production of optical fiber preforms, theconformity tests, vis-à-vis the standards, are carried out by takinginto account only significant figures indicated in the standard. Thus,when the G.657B standard imposes the limit value of 0.01 dB/turn at awavelength of 1625 nanometers for a radius of curvature of 15millimeters, the manufacturer will tolerate bending losses ranging up to0.014 dB/turn at this wavelength for this radius of curvature. Bendinglosses less than 1.5×10⁻³ dB/turn for a radius of curvature of 15millimeters at a wavelength of 1625 nanometers, such as can be providedby the fiber according to the present invention, are therefore at leastten times better than the limits imposed by the standard.

The column FOM of Table IV shows that the fibers of the invention haveclearly improved bending losses relative to the existing BIF fibers,which correspond to the requirements of the G.657B standard.

The fibers disclosed herein are well suited to a use in optical systemsinstalled in the subscriber's home (e.g., FTTH deployments) in which thefiber is subjected to significant bend stresses due to theminiaturization of the optical box or holding the fiber in place withstaples. The present fiber can be placed in particularly compact opticalboxes. In fact, the optical fiber may be arranged with a radius ofcurvature of less than 15 millimeters (e.g., a radius of curvature about5 millimeters). The fiber remains compatible with the fibers of existingsystems, particularly with respect to mode field diameter for goodfiber-to-fiber coupling. The increase in the cut-off wavelength is notdetrimental due to a significant attenuation of the LP11 mode from awavelength of 1260 nanometers.

As set forth in commonly assigned U.S. Patent Application No. 60/986,737for a Microbend-Resistant Optical Fiber (Overton), commonly assignedU.S. Patent Application No. 61/041,484 (Overton) for aMicrobend-Resistant Optical Fiber, and commonly assigned U.S. PatentApplication No. 61/112,595 (Overton) for a Microbend-Resistant OpticalFiber, pairing a bend-insensitive glass fiber (e.g., Draka Comteq'ssingle-mode glass fibers available under the trade nameBendBright^(XS)®) and a primary coating having very low modulus (e.g.,DSM Desotech's UV-curable urethane acrylate product provided under thetrade name DeSolite® DP 1011) achieves optical fibers havingexceptionally low losses (e.g., reductions in microbend sensitivity ofat least 10× as compared with a single-mode fiber employing aconventional coating system). It is further within the scope of thepresent invention to employ the coatings disclosed in U.S. PatentApplication No. 60/986,737, U.S. Patent Application No. 61/041,484, andU.S. Patent Application No. 61/112,595 with the single-mode opticalfiber of the present invention.

Accordingly, this application incorporates entirely by reference thefollowing commonly assigned patent applications: U.S. Patent ApplicationNo. 60/986,737 for a Microbend-Resistant Optical Fiber, filed Nov. 9,2007, (Overton); U.S. Patent Application No. 61/041,484 for aMicrobend-Resistant Optical Fiber, filed Apr. 1, 2008, (Overton); U.S.Patent Application No. 61/112,595 for a Microbend-Resistant OpticalFiber, filed Nov. 7, 2008, (Overton); International Patent ApplicationNo. PCT/U.S.08/82927 Microbend-Resistant Optical Fiber, filed Nov. 9,2008, (Overton); and U.S. patent application Ser. No. 12/267,732 for aMicrobend-Resistant Optical Fiber, filed Nov. 10, 2008, (Overton).

In this regard, microbending can be analyzed according to the IECfixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B,40-micron grade sandpaper), which provides a microbending stresssituation that affects single-mode fibers even at room temperature. TheIEC TR62221 microbending-sensitivity technical report and standard testprocedures (e.g., IEC TR62221, Method B (fixed-diameter sandpaper drum)and Method D (basketweave)) are hereby incorporated by reference intheir entirety.

This application further incorporates entirely by reference thefollowing commonly assigned patents, patent application publications,and patent applications, each of which discusses optical fibers: U.S.Pat. No. 4,838,643 for a Single Mode Bend Insensitive Fiber for Use inFiber Optic Guidance Applications (Hodges et al.); U.S. PatentApplication Publication No. US2007/0127878 A1 and its related U.S.patent application Ser. No. 11/556,895 for a Single Mode Optical Fiber(de Montmorillon et al.); U.S. Patent Application Publication No.US2007/0280615 A1 and its related U.S. patent application Ser. No.11/697,994 for a Single-Mode Optical Fiber (de Montmorillon et al.);U.S. Pat. No. 7,356,234 for Chromatic Dispersion Compensating Fiber (deMontmorillon et al.); U.S. Pat. No. 7,483,613 for Chromatic DispersionCompensating Fiber (de Montmorillon et al.); U.S. Patent ApplicationPublication No. US2008/0152288 A1 and its related U.S. patentapplication Ser. No. 11/999,333 for an Optical Fiber (Flammer et al.);U.S. patent application Ser. No. 12/098,804 for Transmission OpticalFiber Having Large Effective Area (Sillard et al.); U.S. patentapplication Ser. No. 12/418,523 for Dispersion-Shifted Optical Fiber(Sillard et al.); U.S. Patent Application No. 61/101,337 for aBend-Insensitive Optical Fiber, filed Sep. 30, 2008, (de Montmorillon etal.); U.S. Patent Application No. 61/112,006 for a Bend-InsensitiveSingle-Mode Optical Fiber, filed Nov. 6, 2008, (de Montmorillon et al.);U.S. Patent Application No. 61/112,374 for a Bend-InsensitiveSingle-Mode Optical Fiber, filed Nov. 7, 2008, (de Montmorillon et al.);and U.S. patent application Ser. No. ______ for a Bend-InsensitiveSingle Mode Optical Fiber, concurrently filed May 6, 2009, (deMontmorillon et al.)

The present fibers may facilitate the reduction in overall optical-fiberdiameter. As will be appreciated by those having ordinary skill in theart, a reduced-diameter optical fiber is cost-effective, requiring lessraw material. Moreover, a reduced-diameter optical fiber requires lessdeployment space (e.g., within a buffer tube and/or fiber optic cable),thereby facilitating increased fiber count and/or reduced cable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fiber mayhave an outer diameter of about 125 microns. With respect to the opticalfiber's surrounding coating layers, the primary coating may have anouter diameter of between about 175 microns and about 195 microns (i.e.,a primary coating thickness of between about 25 microns and 35 microns)and the secondary coating may have an outer diameter of between about235 microns and about 265 microns (i.e., a secondary coating thicknessof between about 20 microns and 45 microns). Optionally, the presentoptical fiber may include an outermost ink layer, which is typicallybetween two and ten microns.

In an alternative embodiment, the present optical fiber may possess areduced diameter (e.g., an outermost diameter between about 150 micronsand 230 microns). In this alternative optical fiber configuration, thethickness of the primary coating and/or secondary coating is reduced,while the diameter of the component glass fiber is maintained at about125 microns. By way of example, in such embodiments the primary coatinglayer may have an outer diameter of between about 135 microns and about175 microns (e.g., about 160 microns), and the secondary coating layermay have an outer diameter of between about 150 microns and about 230microns (e.g., more than about 165 microns, such as 190-210 microns orso). In other words, the total diameter of the optical fiber is reducedto less than about 230 microns (e.g., about 200 microns).

As noted, the present optical fiber may include one or more coatinglayers (e.g., a primary coating and a secondary coating). At least oneof the coating layers—typically the secondary coating—may be coloredand/or possess other markings to help identify individual fibers.Alternatively, a tertiary ink layer may surround the primary andsecondary coatings.

The present optical fiber may be deployed in various structures, such asthose exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosedwithin a buffer tube. For instance, optical fiber may be deployed ineither a single fiber loose buffer tube or a multi-fiber loose buffertube. With respect to the latter, multiple optical fibers may be bundledor stranded within a buffer tube or other structure. In this regard,within a multi-fiber loose buffer tube, fiber sub-bundles may beseparated with binders (e.g., each fiber sub-bundle is enveloped in abinder). Moreover, fan-out tubing may be installed at the termination ofsuch loose buffer tubes to directly terminate loose buffered opticalfibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermostoptical fiber coating (i.e., tight buffered fiber) or otherwise surroundthe outermost optical fiber coating or ink layer to provide an exemplaryradial clearance of between about 50 and 100 microns (i.e., a semi-tightbuffered fiber).

With respect to the former tight buffered fiber, the buffering may beformed by coating the optical fiber with a curable composition (e.g., aUV-curable material) or a thermoplastic material. The outer diameter oftight buffer tubes, regardless of whether the buffer tube is formed froma curable or non-curable material, is typically less about 1,000 microns(e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may beincluded between the optical fiber and the buffer tube (e.g., to providea gliding layer).

As will be known by those having ordinary skill in the art, an exemplarybuffer tube enclosing optical fibers as disclosed herein may be formedof polyolefins (e.g., polyethylene or polypropylene), includingfluorinated polyolefins, polyesters (e.g., polybutylene terephthalate),polyamides (e.g., nylon), as well as other polymeric materials andblends. In general, a buffer tube may be formed of one or more layers.The layers may be homogeneous or include mixtures or blends of variousmaterials within each layer.

In this context, the buffer tube may be extruded (e.g., an extrudedpolymeric material) or pultruded (e.g., a pultruded, fiber-reinforcedplastic). By way of example, the buffer tube may include a material toprovide high temperature and chemical resistance (e.g., an aromaticmaterial or polysulfone material).

Although buffer tubes typically have a circular cross section, buffertubes alternatively may have an irregular or non-circular shape (e.g.,an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply besurrounded by an outer protective sheath or encapsulated within a sealedmetal tube. In either structure, no intermediate buffer tube isnecessarily required.

Multiple optical fibers as disclosed herein may be sandwiched,encapsulated, and/or edge bonded to form an optical fiber ribbon.Optical fiber ribbons can be divisible into subunits (e.g., atwelve-fiber ribbon that is splittable into six-fiber subunits).Moreover, a plurality of such optical fiber ribbons may be aggregated toform a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or aribbon stack in which the uppermost and lowermost optical fiber ribbonshave fewer optical fibers than those toward the center of the stack.This construction may be useful to increase the density of opticalelements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmissionelements in buffer tubes or cables, subject to other constraints (e.g.,cable or mid-span attenuation). The optical elements themselves may bedesigned for increased packing density. For example, the optical fibermay possess modified properties, such as improved refractive-indexprofile, core or cladding dimensions, or primary coating thicknessand/or modulus, to improve microbending and macrobendingcharacteristics.

By way of example, a rectangular ribbon stack may be formed with orwithout a central twist (i.e., a “primary twist”). Those having ordinaryskill in the art will appreciate that a ribbon stack is typicallymanufactured with rotational twist to allow the tube or cable to bendwithout placing excessive mechanical stress on the optical fibers duringwinding, installation, and use. In a structural variation, a twisted (oruntwisted) rectangular ribbon stack may be further formed into acoil-like configuration (e.g., a helix) or a wave-like configuration(e.g., a sinusoid). In other words, the ribbon stack may possess regular“secondary” deformations.

As will be known to those having ordinary skill in the art, such opticalfiber ribbons may be positioned within a buffer tube or othersurrounding structure, such as a buffer-tube-free cable. Subject tocertain restraints (e.g., attenuation) it is desirable to increase thedensity of elements such as optical fibers or optical fiber ribbonswithin buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be positioned externally adjacent to and strandedaround a central strength member. This stranding can be accomplished inone direction, helically, known as “S” or “Z” stranding, or ReverseOscillated Lay stranding, known as “S-Z” stranding. Stranding about thecentral strength member reduces optical fiber strain when cable strainoccurs during installation and use.

Those having ordinary skill in the art will understand the benefit ofminimizing fiber strain for both tensile cable strain and longitudinalcompressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur duringinstallation, the cable will become longer while the optical fibers canmigrate closer to the cable's neutral axis to reduce, if not eliminate,the strain being translated to the optical fibers. With respect tolongitudinal compressive strain, which may occur at low operatingtemperatures due to shrinkage of the cable components, the opticalfibers will migrate farther away from the cable's neutral axis toreduce, if not eliminate, the compressive strain being translated to theoptical fibers.

In a variation, two or more substantially concentric layers of buffertubes may be positioned around a central strength member. In a furthervariation, multiple stranding elements (e.g., multiple buffer tubesstranded around a strength member) may themselves be stranded aroundeach other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers(e.g., loose or ribbonized fibers) may be simply placed externallyadjacent to the central strength member (i.e., the buffer tubes are notintentionally stranded or arranged around the central strength member ina particular manner and run substantially parallel to the centralstrength member).

Alternatively still, the present optical fibers may be positioned withina central buffer tube (i.e., the central buffer tube cable has a centralbuffer tube rather than a central strength member). Such a centralbuffer tube cable may position strength members elsewhere. For instance,metallic or non-metallic (e.g., GRP) strength members may be positionedwithin the cable sheath itself, and/or one or more layers ofhigh-strength yarns (e.g., aramid or non-aramid yarns) may be positionedparallel to or wrapped (e.g., contrahelically) around the central buffertube (i.e., within the cable's interior space). Likewise, strengthmembers can be included within the buffer tube's casing.

In other embodiments, the optical fibers may be placed within a slottedcore cable. In a slotted core cable, optical fibers, individually or asa fiber ribbon, may be placed within pre-shaped helical grooves (i.e.,channels) on the surface of a central strength member, thereby forming aslotted core unit. The slotted core unit may be enclosed by a buffertube. One or more of such slotted core units may be placed within aslotted core cable. For example, a plurality of slotted core units maybe helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitubecable design, whereby the optical fibers are stranded around themselveswithin a large multi-fiber loose buffer tube rather than around acentral strength member. In other words, the large multi-fiber loosebuffer tube is centrally positioned within the maxitube cable. Forexample, such maxitube cables may be deployed in optical ground wires(OPGW).

In another cabling embodiment, multiple buffer tubes may be strandedaround themselves without the presence of a central member. Thesestranded buffer tubes may be surrounded by a protective tube. Theprotective tube may serve as the outer casing of the fiber optic cableor may be further surrounded by an outer sheath. The protective tube maytightly or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additionalelements may be included within a cable core. For example, copper cablesor other active, transmission elements may be stranded or otherwisebundled within the cable sheath. Passive elements may also be placedwithin the cable core, such as between the interior walls of the buffertubes and the enclosed optical fibers. Alternatively and by way ofexample, passive elements may be placed outside the buffer tubes betweenthe respective exterior walls of the buffer tubes and the interior wallof the cable jacket, or, within the interior space of a buffer-tube-freecable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or othermaterials containing water-swellable material and/or coated withwater-swellable materials (e.g., including super absorbent polymers(SAPs), such as SAP powder) may be employed to provide water blockingand/or to couple the optical fibers to the surrounding buffer tubeand/or cable jacketing (e.g., via adhesion, friction, and/orcompression). Exemplary water-swellable elements are disclosed incommonly assigned U.S. Patent Application Publication No. US2007/0019915A1 for a Water-Swellable Tape, Adhesive-Backed for Coupling When UsedInside a Buffer Tube (Overton et al.), each of which is herebyincorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive,such as a silicone acrylate cross-linked by exposure to actinicradiation) may be provided on one or more passive elements (e.g.,water-swellable material) to bond the elements to the buffer tube. Anadhesive material may also be used to bond the water-swellable elementto optical fibers within the buffer tube. Exemplary arrangements of suchelements are disclosed in commonly assigned U.S. Patent ApplicationPublication No. US2008/0145010 A1 for a Gel-Free Buffer Tube withAdhesively Coupled Optical Element (Overton et al.), which is herebyincorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain athixotropic composition (e.g., grease or grease-like gels) between theoptical fibers and the interior walls of the buffer tubes. For example,filling the free space inside a buffer tube with water-blocking,petroleum-based filling grease helps to block the ingress of water.Further, the thixotropic filling grease mechanically (i.e., viscously)couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, therebyhindering connection and splicing operations. Thus, the present opticalfibers may be deployed in dry cable structures (i.e., grease-free buffertubes).

Exemplary buffer tube structures that are free from thixotropic fillinggreases are disclosed in commonly assigned U.S. Patent ApplicationPublication No. US2009/0003785 A1 for a Coupling Composition for OpticalFiber Cables (Parris et al.), which is hereby incorporated by referencein its entirety. Such buffer tubes employ coupling compositions formedfrom a blend of high-molecular weight elastomeric polymers (e.g., about35 weight percent or less) and oils (e.g., about 65 weight percent ormore) that flow at low temperatures. Unlike thixotropic filling greases,the coupling composition (e.g., employed as a cohesive gel or foam) istypically dry and, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cableenclosing optical fibers as disclosed herein may have a sheath formedfrom various materials in various designs. Cable sheathing may be formedfrom polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectricmaterial (e.g., non-conducting polymers), with or without supplementalstructural components that may be used to improve the protection (e.g.,from rodents) and strength provided by the cable sheath. For example,one or more layers of metallic (e.g., steel) tape along with one or moredielectric jackets may form the cable sheathing. Metallic or fiberglassreinforcing rods (e.g., GRP) may also be incorporated into the sheath.In addition, aramid, fiberglass, or polyester yarns may be employedunder the various sheath materials (e.g., between the cable sheath andthe cable core), and/or ripcords may be positioned, for example, withinthe cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have acircular cross section, but cable sheaths alternatively may have anirregular or non-circular shape (e.g., an oval, trapezoidal, or flatcross-section).

By way of example, the present optical fiber may be incorporated intosingle-fiber drop cables, such as those employed for Multiple DwellingUnit (MDU) applications. In such deployments, the cable jacketing mustexhibit crush resistance, abrasion resistance, puncture resistance,thermal stability, and fire resistance as required by building codes. Anexemplary material for such cable jackets is thermally stable,flame-retardant polyurethane (PUR), which mechanically protects theoptical fibers yet is sufficiently flexible to facilitate easy MDUinstallations. Alternatively, a flame-retardant polyolefin or polyvinylchloride sheath may be used.

In general and as will be known to those having ordinary skill in theart, a strength member is typically in the form of a rod orbraided/helically wound wires or fibers, though other configurationswill be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may bevariously deployed, including as drop cables, distribution cables,feeder cables, trunk cables, and stub cables, each of which may havevarying operational requirements (e.g., temperature range, crushresistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts,plenums, or risers. By way of example, an optical fiber cable may beinstalled in an existing duct or microduct by pulling or blowing (e.g.,using compressed air). An exemplary cable installation method isdisclosed in commonly assigned U.S. Patent Application Publication No.US2007/0263960 for a Communication Cable Assembly and InstallationMethod (Lock et al.), and U.S. Patent Application Publication No.US2008/0317410 for a Modified Pre-Ferrulized Communication CableAssembly and Installation Method (Griffioen et al.), each of which isincorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be stranded (e.g., around a central strengthmember). In such configurations, an optical fiber cable's protectiveouter sheath may have a textured outer surface that periodically varieslengthwise along the cable in a manner that replicates the strandedshape of the underlying buffer tubes. The textured profile of theprotective outer sheath can improve the blowing performance of theoptical fiber cable. The textured surface reduces the contact surfacebetween the cable and the duct or microduct and increases the frictionbetween the blowing medium (e.g., air) and the cable. The protectiveouter sheath may be made of a low coefficient-of-friction material,which can facilitate blown installation. Moreover, the protective outersheath can be provided with a lubricant to further facilitate blowninstallation.

In general, to achieve satisfactory long-distance blowing performance(e.g., between about 3,000 to 5,000 feet or more), the outer cablediameter of an optical fiber cable should be no more than about seventyto eighty percent of the duct's or microducts inner diameter.

Compressed air may also be used to install optical fibers in an airblown fiber system. In an air blown fiber system, a network of unfilledcables or microducts is installed prior to the installation of opticalfibers. Optical fibers may subsequently be blown into the installedcables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the groundor, as an aerial cable, suspended from a pole or pylon. An aerial cablemay be self-supporting, or secured or lashed to a support (e.g.,messenger wire or another cable). Exemplary aerial fiber optic cablesinclude overhead ground wires (OPGW), all-dielectric self-supportingcables (ADSS), all dielectric lash cables (AD-Lash), and figure-eightcables, each of which is well understood by those having ordinary skillin the art. (Figure-eight cables and other designs can be directlyburied or installed into ducts, and may optionally include a toningelement, such as a metallic wire, so that they can be found with a metaldetector.

In addition, although the optical fibers may be further protected by anouter cable sheath, the optical fiber itself may be further reinforcedso that the optical fiber may be included within a breakout cable, whichallows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmissionsystem, connections are required at various points in the network.Optical fiber connections are typically made by fusion splicing,mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the fiber ends eitherin the field (e.g., at the network location) or in a factory prior toinstallation into the network. The ends of the connectors are mated inthe field in order to connect the fibers together or connect the fibersto the passive or active components. For example, certain optical fibercable assemblies (e.g., furcation assemblies) can separate and conveyindividual optical fibers from a multiple optical fiber cable toconnectors in a protective manner.

The deployment of such optical fiber cables may include supplementalequipment, which itself may employ the present optical fiber aspreviously disclosed. For instance, an amplifier may be included toimprove optical signals. Dispersion compensating modules may beinstalled to reduce the effects of chromatic dispersion and polarizationmode dispersion. Splice boxes, pedestals, and distribution frames, whichmay be protected by an enclosure, may likewise be included. Additionalelements include, for example, remote terminal switches, optical networkunits, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use ina communication system (e.g., networking or telecommunications). Acommunication system may include fiber optic cable architecture such asfiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure(FTTE), fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), andfiber-to-the-home (FTTH), as well as long-haul or metro architecture.Moreover, an optical module or a storage box that includes a housing mayreceive a wound portion of the optical fiber disclosed herein. By way ofexample, the optical fiber may be wound with a bending radius of lessthan about 15 millimeters (e.g., 10 millimeters or less, such as about 5millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications,including, without limitation, fiber optic sensors or illuminationapplications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). Aswill be known by those having ordinary skill in the art, FBG is aperiodic or aperiodic variation in the refractive index of an opticalfiber core and/or cladding. This variation in the refractive indexresults in a range of wavelengths (e.g., a narrow range) being reflectedrather than transmitted, with maximum reflectivity occurring at theBragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber byexposing the optical fiber to an intense source of ultraviolet light(e.g., a UV laser). In this respect, UV photons may have enough energyto break molecular bonds within an optical fiber, which alters thestructure of the fiber, thereby increasing the fiber's refractive index.Moreover, dopants (e.g., boron or germanium) and/or hydrogen loading canbe employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation ofFBG, the coating may be removed. Alternatively, coatings that aretransparent at the particular UV wavelengths (e.g., the UV wavelengthsemitted by a UV laser to write FBG) may be employed to render coatingremoval unnecessary. In addition, silicone, polyimide, acrylate, or PFCBcoatings, for instance, may be employed for high-temperatureapplications.

A particular FBG pattern may be created by employing (i) a photomaskplaced between the UV light source and the optical fiber, (ii)interference between multiple UV light beams, which interfere with eachother in accordance with the desired FBG pattern (e.g., a uniform,chirped, or titled pattern), or (iii) a narrow UV light beam forcreating individual variations. The FBG structure may have, for example,a uniform positive-only index change, a Gaussian-apodized index change,a raised-cosine-apodized index change, or a discrete phase shift indexchange. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensingapplications (e.g., for detecting vibration, temperature, pressure,moisture, or movement). In this respect, changes in the optical fiber(e.g., a change in temperature) result in a shift in the Braggwavelength, which is measured by a sensor. FBG may be used to identify aparticular optical fiber (e.g., if the fiber is broken into pieces).

Fiber Bragg Grating may also be used in various active or passivecommunication components (e.g., wavelength-selective filters,multiplexers, demultiplexers, Mach-Zehnder interferometers, distributedBragg reflector lasers, pump/laser stabilizers, and supervisorychannels).

This application further incorporates entirely by reference thefollowing commonly assigned patents, patent application publications,and patent applications: U.S. Pat. No. 5,574,816 forPolypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,717,805 forStress Concentrations in an Optical Fiber Ribbon to FacilitateSeparation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362 forPolypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,911,023 forPolyolefin Materials Suitable for Optical Fiber Cable Components; U.S.Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.6,035,087 for Optical Unit for Fiber Optic Cables; U.S. Pat. No.6,066,397 for Polypropylene Filler Rods for Optical Fiber CommunicationsCables; U.S. Pat. No. 6,175,677 for Optical Fiber Multi-Ribbon andMethod for Making the Same; U.S. Pat. No. 6,085,009 for Water BlockingGels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes andCables Made Therewith; U.S. Pat. No. 6,215,931 for FlexibleThermoplastic Polyolefin Elastomers for Buffering Transmission Elementsin a Telecommunications Cable; U.S. Pat. No. 6,134,363 for Method forAccessing Optical Fibers in the Midspan Region of an Optical FiberCable; U.S. Pat. No. 6,381,390 for Color-Coded Optical Fiber Ribbon andDie for Making the Same; U.S. Pat. No. 6,181,857 for Method forAccessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224for Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section;U.S. Pat. No. 6,334,016 for Optical Fiber Ribbon Matrix Material HavingOptimal Handling Characteristics; U.S. Pat. No. 6,321,012 for OpticalFiber Having Water Swellable Material for Identifying Grouping of FiberGroups; U.S. Pat. No. 6,321,014 for Method for Manufacturing OpticalFiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene Filler Rods forOptical Fiber Communications Cables; U.S. Pat. No. 6,493,491 for OpticalDrop Cable for Aerial Installation; U.S. Pat. No. 7,346,244 for CoatedCentral Strength Member for Fiber Optic Cables with Reduced Shrinkage;U.S. Pat. No. 6,658,184 for Protective Skin for Optical Fibers; U.S.Pat. No. 6,603,908 for Buffer Tube that Results in Easy Access to andLow Attenuation of Fibers Disposed Within Buffer Tube; U.S. Pat. No.7,045,010 for Applicator for High-Speed Gel Buffering of FlextubeOptical Fiber Bundles; U.S. Pat. No. 6,749,446 for Optical Fiber Cablewith Cushion Members Protecting Optical Fiber Ribbon Stack; U.S. Pat.No. 6,922,515 for Method and Apparatus to Reduce Variation of ExcessFiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No.6,618,538 for Method and Apparatus to Reduce Variation of Excess FiberLength in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No. 7,322,122for Method and Apparatus for Curing a Fiber Having at Least Two FiberCoating Curing Stages; U.S. Pat. No. 6,912,347 for Optimized Fiber OpticCable Suitable for Microduct Blown Installation; U.S. Pat. No. 6,941,049for Fiber Optic Cable Having No Rigid Strength Members and a ReducedCoefficient of Thermal Expansion; U.S. Pat. No. 7,162,128 for Use ofBuffer Tube Coupling Coil to Prevent Fiber Retraction; U.S. Pat. No.7,515,795 for a Water-Swellable Tape, Adhesive-Backed for Coupling WhenUsed Inside a Buffer Tube (Overton et al.); International PatentApplication Publication No. 2007/013923 for Grease-Free Buffer OpticalFiber Buffer Tube Construction Utilizing a Water-Swellable, TexturizedYarn (Overton et al.); European Patent Application Publication No.1,921,478 A1, for a Telecommunication Optical Fiber Cable (Tatat etal.); U.S. Patent Application Publication No. US2007/0183726 A1 for anOptical Fiber Cable Suited for Blown Installation or PushingInstallation in Microducts of Small Diameter (Nothofer et al.); U.S.Patent Application Publication No. US 2008/0037942 A1 for an OpticalFiber Telecommunications Cable (Tatat); U.S. Patent ApplicationPublication No. US2008/0145010 A1 for a Gel-Free Buffer Tube withAdhesively Coupled Optical Element (Overton et al.); U.S. PatentApplication Publication No. US2008/0181564 A1 for a Fiber Optic CableHaving a Water-Swellable Element (Overton); U.S. Patent ApplicationPublication No. US2009/0041414 A1 for a Method for Accessing OpticalFibers within a Telecommunication Cable (Lavenne et al.); U.S. PatentApplication Publication No. US2009/0003781 A1 for an Optical Fiber CableHaving a Deformable Coupling Element (Parris et al.); U.S. PatentApplication Publication No. US2009/0003779 A1 for an Optical Fiber CableHaving Raised Coupling Supports (Parris); U.S. Patent ApplicationPublication No. US2009/0003785 A1 for a Coupling Composition for OpticalFiber Cables (Parris et al.); and U.S. patent application Ser. No.12/391,327 for a Buffer Tube with Hollow Channels, filed Feb. 24, 2009,(Lookadoo et al.).

In the specification and/or figures, typical embodiments of theinvention have been disclosed. The present invention is not limited tosuch exemplary embodiments. The figures are schematic representationsand so are not necessarily drawn to scale. Unless otherwise noted,specific terms have been used in a generic and descriptive sense and notfor purposes of limitation.

1. A single-mode optical fiber, comprising: a central core surrounded byan outer cladding, the central core having a radius r₁ and a positiverefractive index difference Δn₁ with the optical cladding; anintermediate cladding positioned between the central core and the outercladding, the intermediate cladding having a radius r₂ and a positiverefractive index difference Δn₂ with the optical cladding, wherein therefractive index difference Δn₂ is less than the refractive indexdifference Δn₁; a depressed trench positioned between the intermediatecladding and the outer cladding, the depressed trench having a radius r₃and a negative refractive index difference Δn₃ with the opticalcladding; wherein, at a wavelength of 1310 nanometers, the optical fiberhas a mode field diameter (MFD) between 8.6 microns and 9.5 microns;wherein, at a wavelength of 1550 nanometers, the optical fiber hasbending losses less than 0.25×10⁻³ dB/turn for a radius of curvature of15 millimeters; and wherein, at a wavelength of 1260 nanometers, theLP11 mode of the optical fiber is attenuated to 19.3 dB at a length lessthan 90 meters.
 2. The optical fiber according to claim 1, wherein thesurface integral of the central core (V₀₁), defined asV₀₁ = ∫₀^(r 1)Δ n(r)⋅ r ≈ r₁ × Δ n₁, is between 20.0×10⁻³micron and 23.0×10⁻³ micron.
 3. The optical fiber according to claim 1,wherein the surface integral of the depressed trench (V₀₃), defined asV₀₃ = ∫_(r 2)^(r 3)Δ n(r)⋅ r ≈ (r₃ − r₂) × Δ n₃, is between−55.0×10⁻³ micron and −30.0×10⁻³ micron.
 4. The optical fiber accordingto claim 1, wherein the volume integral of the depressed trench (V₁₃),defined asV₁₃ = 2 ⋅ ∫_(r 2)^(r 3)Δ n(r)⋅ r ⋅ r ≈ (r₃² − r₂²) × Δ n₃,is between −1200×10⁻³ μm² and −750×10⁻³ μm².
 5. The optical fiberaccording to claim 1, wherein the optical fiber has an effective cut-offwavelength λ_(ceff) greater than 1350 nanometers, the effective cut-offwavelength λ_(ceff) being measured as the wavelength at which theoptical signal becomes single mode after propagation over two meters offiber.
 6. The optical fiber according to claim 1, wherein, at awavelength of 1550 nanometers, the optical fiber has bending losses lessthan or equal to 7.5×10⁻³ dB/turn for a radius of curvature of 10millimeters.
 7. The optical fiber according to claim 1, wherein, at awavelength of 1550 nanometers, the optical fiber has bending losses lessthan or equal to 0.05 dB/turn for a radius of curvature of 7.5millimeters.
 8. The optical fiber according to claim 1, wherein, at awavelength of 1550 nanometers, the optical fiber has bending losses lessthan 0.15 dB/turn for a radius of curvature of 5 millimeters.
 9. Theoptical fiber according to claim 1, wherein, at a wavelength of 1625nanometers, the optical fiber has bending losses less than 1.5×10⁻³dB/turn for a radius of curvature of 15 millimeters.
 10. The opticalfiber according to claim 1, wherein, at a wavelength of 1625 nanometers,the optical fiber has bending losses less than or equal to 25×10⁻³dB/turn for a radius of curvature of 10 millimeters.
 11. The opticalfiber according to claim 1, wherein, at a wavelength of 1625 nanometers,the optical fiber has bending losses less than or equal to 0.08 dB/turnfor a radius of curvature of 7.5 millimeters.
 12. The optical fiberaccording to claim 1, wherein, at a wavelength of 1625 nanometers, theoptical fiber has bending losses less than 0.25 dB/turn for a radius ofcurvature of 5 millimeters.
 13. The optical fiber according to claim 1,wherein, at a wavelength of 1260 nanometers, the LP11 mode of theoptical fiber is attenuated to 19.3 dB at a length less than about 40meters.
 14. The optical fiber according to claim 1, wherein the opticalfiber has a cut-off wavelength between 1300 nanometers and 1400nanometers.
 15. The optical fiber according to claim 1, wherein theoptical fiber has a cable cut-off wavelength between 1250 nanometers and1300 nanometers, the cable cut-off wavelength being the wavelengthbeyond which the attenuation of the LP11 mode is greater than or equalto 19.3 dB after propagation over 22 meters of fiber.
 16. The opticalfiber according to claim 1, wherein the optical fiber has a theoreticalcut-off wavelength less than or equal to 1250 nanometers, thetheoretical cut-off wavelength being the wavelength from which the LP11mode is propagated in leaky mode.
 17. The optical fiber according toclaim 1, wherein the optical fiber exhibits an attenuation of the LP11mode greater than 5 dB after propagation over 22 meters of fiber at awavelength of 1260 nanometers.
 18. The optical fiber according to claim1, wherein the central core has a radius (r₁) between 3.8 microns and4.35 microns.
 19. The optical fiber according to claim 1, wherein theintermediate cladding has a radius (r₂) between 8.5 microns and 9.7microns.
 20. The optical fiber according to claim 1, wherein thedepressed trench has a radius (r₃) between 13.5 microns and 16 microns.21. The optical fiber according to claim 20, wherein the depressedtrench has a radius (r₃) less than or equal to 15 microns.
 22. Theoptical fiber according to claim 1, wherein the central core has arefractive index difference (Δn₁) with the optical cladding between5.3×10⁻³ and 5.7×10⁻³.
 23. The optical fiber according to claim 1,wherein the intermediate cladding has a refractive index difference(Δn₂) with the optical cladding between 0.1×10⁻³ and 0.6×10⁻³.
 24. Theoptical fiber according to claim 1, wherein the depressed trench has arefractive index difference (Δn₃) with the optical cladding between−10.0×10⁻³ and −5.0×10⁻³.
 25. The optical fiber according claim 1,wherein the optical fiber has a zero chromatic dispersion wavelength(ZDW) between 1300 nanometers and 1324 nanometers.
 26. The optical fiberaccording to claim 1, wherein the optical fiber has a zero chromaticdispersion slope value (ZDS) at the chromatic zero dispersion wavelengthless than 0.092 ps/(nm²·km).
 27. A buffer tube containing one or moreoptical fibers according to claim
 1. 28. A cable containing one or moreoptical fibers according to claim
 1. 29. An optical box receiving atleast a portion of the optical fiber according to claim
 1. 30. Anoptical box according to claim 29, wherein the optical fiber ispositioned within the optical box such that the optical fiber has aradius of curvature less than 15 millimeters.
 31. An optical boxaccording to claim 29, wherein the optical fiber is positioned withinthe optical box such that the optical fiber has a radius of curvatureless than 5 millimeters.
 32. A Fiber-To-The-Home (FTTH) systemcomprising at least a portion of the optical fiber according to claim 1.33. A single-mode optical fiber, comprising: a central core surroundedby an outer cladding, the central core having a radius r₁ and arefractive index difference Δn₁ with the optical cladding between about5.3×10⁻³ and 5.7×10⁻³; an intermediate cladding positioned between thecentral core and the outer cladding, the intermediate cladding having aradius r₂ and a refractive index difference Δn₂ with the opticalcladding between about 0.1×10⁻³ and 0.6×10⁻³; a depressed trenchpositioned between the intermediate cladding and the outer cladding, thedepressed trench having a radius r₃ and a refractive index differenceΔn₃ with the optical cladding between about −10.0×10⁻³ and −5.0×10⁻³;wherein, at a wavelength of 1310 nanometers, the optical fiber has amode field diameter (MFD) between about 8.6 microns and 9.5 microns;wherein the optical fiber has a zero chromatic dispersion wavelength(ZDW) between 1300 nanometers and 1324 nanometers; wherein the opticalfiber has a zero chromatic dispersion slope value (ZDS) at the chromaticzero dispersion wavelength less than 0.092 ps/(nm²·km); and wherein, ata wavelength of 1260 nanometers, the LP11 mode of the optical fiber isattenuated to 19.3 dB at a length less than 90 meters.
 34. The opticalfiber according to claim 33, wherein: the surface integral of thecentral core (V₀₁), defined asV₀₁ = ∫₀^(r 1)Δ n(r)⋅ r ≈ r₁ × Δ n₁, is between 20.0×10⁻³micron and 23.0×10⁻³ micron; the surface integral of the depressedtrench (V₀₃), defined asV₀₃ = ∫_(r 2)^(r 3)Δ n(r)⋅ r ≈ (r₃ − r₂) × Δ n₃, is between−55.0×10⁻³ micron and −30.0×10⁻³ micron; and the volume integral of thedepressed trench (V₁₃), defined asV₁₃ = 2 ⋅ ∫_(r 2)^(r 3)Δ n(r)⋅ r ⋅ r ≈ (r₃² − r₂²) × Δ n₃,is between −1200×10⁻³ μm² and −750×10⁻³ μm².
 35. The optical fiberaccording to claim 33, wherein, at a wavelength of 1260 nanometers, theLP11 mode of the optical fiber is attenuated to 19.3 dB at a length lessthan about 50 meters.
 36. The optical fiber according to claim 33,wherein the central core has a radius (r₁) between about 3.8 microns and4.35 microns.
 37. The optical fiber according to claim 33, wherein theintermediate cladding has a radius (r₂) between about 8.5 microns and9.7 microns.
 38. The optical fiber according to claim 33, wherein thedepressed trench has a radius (r₃) less than about 16 microns.